Making an atomic bomb requires skill, guts

WASHINGTON—Making an atomic bomb isn't for dummies—or for wimps. It takes a wide set of technical capabilities and a willingness to risk death or injury to join the club of nuclear nations.

"It's not done in your basement or bathtub," said Robert Norris, a nuclear weapons expert at the Natural Resources Defense Council, an environmental group in Washington. "You need physicists, chemists, engineers, metallurgists—a whole stable of people with skills and central direction—to make a successful bomb."

There are difficulties each step of the way, from designing the weapon to assembling the dangerous radioactive materials to manufacturing a complicated, high-precision device, delivering it to its target and setting it off at the right time and place.

A modern atomic bomb has to be small and light enough to fit on a missile or in an artillery shell or a suitcase. It can't be as massive as the 5-ton "Fat Man," which an Air Force B-29 bomber dropped on Nagasaki, Japan, in 1945.

Safety is also a concern. Powerful chemical explosives are needed to detonate a nuclear bomb. Weapons-grade plutonium and highly enriched uranium are lethally radioactive.

"This high explosive stuff is dangerous to make," Norris said. "If it's ignited before it's supposed to, it can spread plutonium all over the place. There have been serious nuclear-weapons accidents in the United States."

The easiest bomb to make uses nuclear fission, which splits a mass of uranium or plutonium atoms to release enormous amounts of heat and energy. It's much harder to construct a fusion, or hydrogen, bomb, which fuses atoms together as happens inside the sun.

North Korea, and possibly Iran, is working on fission weapons. North Korea is farther along the learning curve than Iran, as shown by what may have been a partially successful bomb test Monday. Iran would still need to produce enough enriched uranium for a nuclear bomb.

Both countries already have passed the first hurdle: They reportedly have obtained detailed bomb designs from Pakistan's rogue nuclear-weapons expert, Abdul Qadeer Khan.

The next crucial step is to acquire a sufficient stock of nuclear fuel. At least 2 pounds of weapons-grade plutonium or 4 pounds of highly enriched uranium are needed to set off a single fission bomb, according to the Bulletin of the Atomic Scientists.

The form of uranium found in nature, known as U-238, is plentiful, but less than 1 percent of it is U-235, the highly radioactive form needed for a bomb. An elaborate process is required to extract U-235 from U-238.

First, uranium ore, or "yellowcake," is dissolved in nitric acid and converted to a toxic gas known as hexafluoride. The gas is spun in a centrifuge to separate the heavier U-238 from the lighter U-235. The fuel must be enriched until it's at least 90 percent pure. The process takes thousands of centrifuges spinning for months, as North Korea did in the 1990s and Iran reportedly is doing now.

"A few thousand is what you need for one bomb per year," Princeton University nuclear expert Frank von Hippel told the journal Science.

A different procedure is used to make weapons-grade plutonium. Some uranium fuel in a nuclear-power reactor absorbs extra neutrons and gradually turns into plutonium-239. Acids dissolve the uranium in the spent fuel rods and separate out the plutonium.

The most difficult task probably is constructing the bomb itself. It typically consists of a hollow sphere or cylinder in which the explosive fuel is surrounded by a dense material, usually U-238.

The fissionable fuel is very unstable and must be divided into separate "subcritical" masses so it won't blow up prematurely. Two or more of these masses must be rapidly smashed together to sustain a fission chain reaction.

One way to do this in a uranium-fueled bomb is to shoot a small lump of U-235 into a larger piece. This raises the fuel to a critical mass, touching off a spontaneous chain reaction.

Another technique, called "implosion," is used to detonate a plutonium-fueled bomb. A core of plutonium is surrounded by a sphere of little wedge-shaped chemical explosives. When the explosives are ignited they create a shock wave and squeeze the mass until it detonates.

"It's quite complex to fabricate one of these things," Norris said. "You have to get the angles right. It's also dangerous to cut and shape high explosives, which might go off."

Extremely fine-precision machining and complicated electronics are required to make implosion work.

To set off the bomb, the explosive wedges, typically numbering 32 or 64, must all go off at the same instant. Tiny wires leading from each wedge to a central electronic trigger must be exactly the right length to produce a simultaneous explosion. Otherwise the bomb will fizzle, as may have happened in the North Korean test.

"If the plutonium is not squeezed precisely evenly, you get a squirt out of one side and have little or no yield," said Hans Kristensen, the director of the Nuclear Information Project at the Federation of American Scientists.